[priceless]

Ahh -- I think see the issue. You are trying to have a line drawn across a shape, anchored to two magnets within that same shape. Is that right?

When I do that, I also get a line that disappears.

Here's a trick that worked for me: add a third point to the line. As long as it is in line with the other two, it'll look the same and it will work as you expect.

I suspect there is something strange going on -- please also submit a bug to omnigroup using Help --> Send Feedback... from within Omnigraffle.

-- Morgan